1. Field of the Invention
The subject invention generally pertains to sucker rods of sucker rod pumps (typically used in oil wells) and more specifically to a tool system for assembling and disassembling sucker rods.
2. Description of Related Art
Oil wells and many other types of wells often include a sucker rod pump for pumping oil or other fluid from deep within a well bore to the surface of the earth. A sucker rod pump is a reciprocating piston/cylinder type pump situated at the bottom of a long string of tubing that conveys the pumped fluid upward to the earth's surface. An oscillating drive at ground level is coupled to raise and lower the pump's piston by way of long string of sucker rods that may extend over 10,000 feet through the interior of the tubing. The string of sucker rods is comprised of individual solid rods of about 0.5 to 1.125 inches in diameter and about 25 to 30 feet long. Each sucker rod has an axial shoulder and male threads at each end that allow the rods to be tightly connected end-to-end by way of female threaded rod couplings (also referred to as boxes). The couplings also serve as a wear surface that protects the more expensive sucker rod from wear as the string of sucker rods may slide up and down along the interior of the tubing for millions of cycles over its lifetime.
Properly tightening each threaded joint of a string of sucker rods is critically important, as even a single improperly tightened joint can lead to a premature separation, fatigue cracking, or complete breakage of the string. This not only interrupts the ongoing operation of the well, but repairing a string of sucker rods is very expensive, due to its inaccessibility. Usually the entire string of sucker rods is removed from the well bore to repair a single joint. For a 10,000-foot string of 25-foot sucker rods, there are 800 threaded joints. Thus, a reliable system is needed to properly tighten every single one.
Today, power rod tongs are possibly the most common tools for assembling and disassembling a string of sucker rods. Conventional tongs, such as those provided by BJ-Hughes Machinery of Houston, Tex., includes two sets of jaws: one set being driven to rotate relative to the other. To assemble a new joint, a sucker rod is first manually screwed band-tight into each end of a coupling. The rod tong is positioned to engage one set of tong jaws with mating flats of one sucker rod, and the other set of jaws with mating flats of the other sucker rod. This places the coupling generally between, but spaced apart from, the two sets of jaws. Actuating the tong rotates one rod relative to the other, so that both rods screw tightly into the coupling generally at the same time. As the connection tightens, the tong eventually stalls at a torque or pressure preset by the operator. When the tong stalls, the operator assumes that the connection is properly torqued with the proper preload. Thus the operator manually stops the tong and disengages it from the sucker rods.
Controlling torque alone, however, generally disregards several factors that can result in an improperly tightened joint, even though the target torque was reached. Even with sufficient torque, inadequately preload of the joint can result from dry threads, dirt (on the threads or axial faces of the coupling or rod shoulders), galling, and even a cross-wind that causes a rod to sway and bind. Over tightening or excess preload can occur when a tong is not properly calibrated to account for various characteristics of the tong. By monitoring and controlling torque alone, a joint with worn or partially stripped threads may get fully torqued and accepted as a proper joint.
In some instances, monitoring the angular displacement or extent to which one element of a joint is turned relative to another has been successfully applied to achieve a properly tightened joint. When tightening tubing, for example, some tubing tongs include means for monitoring the angular displacement of one tubing section being screwed into an adjoining pipe coupling. This, however, is a simple two-element joint comprising one section of tubing and one pipe coupling. Power tubing tongs with serrated teeth (similar to those of a pipe wrench) can simply bite into the two adjoining elements and control the extent of their relative rotation.
With a three-element joint, such as two sucker rods with a coupling interposed therebetween, such a conventional angular displacement tightening process is impractical for several reasons. Sucker rods are subjected to a tremendous axial load, especially those near the top of the string, as they must support the all the other rods hanging below them. In addition, the raising and lowering of the sucker rods contributes an additional cyclical load that has been known to lead to fatigue cracking at the joints. Consequently, it is desirable to avoid the use of serrated tong jaws whose bite may create detrimental stress concentrations at the joint. Moreover, since many sucker rods have smoothly polished ends to minimize stress concentrations and often have a relatively delicate plastic coating to resist corrosion, it is important to properly engage only the flats of the sucker rods with correspondingly flat jaws as found on conventional rod tongs. This limits the available points of engagement to only certain locations on the sucker rods and restricts one from gripping the rod coupling itself By not biting into the rod coupling of a three-element joint, there becomes a question as to whether one or both rods are being tightened to the coupling.
To settle the question of how many rods are being tightened at one time, a line can be manually scribed on the periphery of the coupling and each rod, and the circumferential displacement of the line can be measured as the three element joint is torqued. However, such a method may only be practical in a test or experimental setting and would be much too time consuming to apply on a regular basis at a field setting. Further, before scribing the line, the shoulder point (i.e., the point at which the shoulders of the sucker rods abut the axial face of an adjoining coupling) would need to be determined, which is not always easy to do accurately.
Sometimes, a properly tightened sucker rod joint can fail after being subjected to averse operating conditions at the well. For example, if a string of sucker rods are driven down faster than the speed at which the rod tends to fall, the string of sucker rods will go into compression at each down stroke. This, of course, will cause the string to bow and thus repeatedly strike the side wall of the tubing. Other dynamic problems include sucker rod resonance and fluid pound or fluid hammering. When such problems causes a joint failure, there becomes a question as to whether the joint was ever properly tightened in the first place. In some cases, joint failures are confined to a particular depth range of the well bore. However, without reliable records of joint make-up during assembly and joint break-out during disassembly, many of the clues that could identify the cause of a particular problem are never discovered.